At present, the energy sector is responsible for about three-fourths of the anthropogenic carbon dioxide emissions. Over the past 15 years, economic activity in the U.S. has increased by about 50%; total U.S. electricity demand has increased 30% over the same period. In the coming years, the surge in the U.S. demand for electric power shows no signs of abating. Economic activity in the U.S. is projected to expand 49% by 2020. Accordingly, in the same period, the demand for electricity is projected to increase by another 30%. With this increase in electricity demand, CO2 emissions from more and more power plants will become an even greater driving force in rising atmospheric temperatures.
Capturing and storing carbon dioxide could slow down climate change and also allow fossil fuels to be a bridge to a clean, renewable energy future. Since the CO2 emitted from electric utilities is the present concern, faster implementation of CO2 capture by chemical means at stationary combustion sources would be highly desirable. While absorption/stripping with amine-based scrubber systems has been successfully used for natural-gas purification, it poses several technical challenges, including the fact that flue gas from utility boilers is at atmospheric pressure and the concentration of CO2 in the flue gas is relatively low at 12-14%. Another technical hurdle is the energy requirements for the CO2 capture/desorption devices to regenerate absorber reagents.
In a typical coal-fired power plant, coal is burned in a boiler to make high temperature and high pressure steam which will drive a steam turbine and generate electricity for power production. After the coal is burned, the gases that are the products of the combustion reaction leave the boiler and are then treated in a NO removal device called a Selective Catalytic Reduction (SCR). After the SCR, the gas is further treated in the Electrostatic Precipitator (ESP) to remove particulate matter. After this, the gas will travel through the SO2 removal device (SO2 scrubber). The next step of the process involves the removal and capture of carbon dioxide and it is this step that is the subject of the present invention.
In accordance with the present invention, an ammonia based solution is used to scrub the flue gas and a membrane is used for purpose of solvent enrichment. Advantageously, the present invention provides a number of benefits that other carbon dioxide capture processes do not possess. First, with the currently available CO2 capture processes, the energy needed to run these processes at coal-fired power plants is very high. Current CO2 capture processes use a monoethanolamine (MEA) solution or other amine-based absorbent solvent solutions. It is estimated that with the current CO2 capture processes, if they were applied to coal-fired flue gas, the cost of electricity would increase by 60-70%. The present invention, using an ammonia solution, will increase the cost of electricity by less than 30%.
Ammonia solutions have been experimented with for years in CO2 capture processes, but ammonia's high vapor pressure is the one glaring negative. Herein lies the second benefit of the present invention. The present invention utilizes a chemical additive that reduces NH3 losses by 80%. This additive also increases the solution reactivity, and reaction kinetics is one of the limiting mechanisms when using NH3 based capture solutions.
The third benefit of the present invention leads to an even smaller increase in electricity costs, compared to amine-based processes. In the present invention process, a membrane is used to reduce the amount of aqueous ammonia and liquid water that is sent to the gas stripper. This lowers the energy penalty even further.